I found a XEP-0198 which can help us to improve messages delivery
It's clear how to use Basic Acking Scenario
But I have some doubts how to use properly Efficient Acking Scenario
For example, user sent 5 messages and then sent r element and received a with h='4' . How he knows which message wasn't delivered to server? which one from 5 messages?
XEP-198 relies on the (byte)stream order guarantee that TCP provides: All messages arrive in-order at the recipient. This means for XEP-198 enabled XMPP connections that the unacknowledged messages are always the last ones.
In your example this means that the last message who got sent of the 5 messages is unacknowledged.